ELECTRICAL CIRCUITRY
First Claim
1. In a combustion control system having a photoconductive photocell for sensing flame a regulated voltage source connected in shunt with said photocell and fail-safe circuitry responsive to changes in said photocell for energizing a relay when said flame is sensed, said fail-safe circuitry including at least one stage of amplification, the improvement comprising a fail-safe regulated voltage source monitoring means, said monitoring means comprising a source of standard voltage, a source of alternating voltage of predetermined magnitude, said monitoring means being arranged to operate in an operative mode when said regulated and standard voltages are within a predetermined range determined by the magnitude of the alternating current source, and a second inoperative mode when said regulated and standard voltages are outside said predetermined range, a solid state device having emitter, base and collector electrodes, means to apply said regulated voltage to said emitter electrode, means including a resistive voltage divider having first and second resistors for applying said standard voltage to said base electrode, means for applying a signal from said source of alternating voltage as an additive component to said standard voltage at said base electrode, means for deriving an output signal from said collector electrode, said solid state device being alternately conductive and cutoff, in operative mode and assuming a single state in inoperative mode, a fail-safe capacitor-diode rectifier circuit connected to the output of said solid state device, said output signal causing said fail-safe rectifier circuit to produce a substantial voltage signal level in said operative mode and to produce a negligible voltage signal level in said inoperative mode, and means for applying said voltage signal level to said amplification stage so that said amplification stage is enabled in response to said substantial voltage signal level and disabled in response to said negligible voltage, said monitoring means assuming said inoperative mode upon failure of any non-resistive component thereof.
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Accused Products
Abstract
The invention describes a fail safe voltage responsive circuit for producing an output level indicative of the relationship between two voltage levels. The voltage responsive circuit includes a fail-safe comparison circuit which assumes alternative modes in response to an alternating signal introduced into the comparison circuit when the voltage levels are within a predetermined range, one relative to the other. When the voltage levels are outside the range or if a component fails, the comparison circuit ceases to alternate between modes. The voltage responsive circuit is used in a preferred application involving a fail-safe combustion control system wherein one voltage level is used to supply bias current to a photoconductive cell, the cell sensing the presence or absence of flame.
8 Citations
10 Claims
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1. In a combustion control system having a photoconductive photocell for sensing flame a regulated voltage source connected in shunt with said photocell and fail-safe circuitry responsive to changes in said photocell for energizing a relay when said flame is sensed, said fail-safe circuitry including at least one stage of amplification, the improvement comprising a fail-safe regulated voltage source monitoring means, said monitoring means comprising a source of standard voltage, a source of alternating voltage of predetermined magnitude, said monitoring means being arranged to operate in an operative mode when said regulated and standard voltages are within a predetermined range determined by the magnitude of the alternating current source, and a second inoperative mode when said regulated and standard voltages are outside said predetermined range, a solid state device having emitter, base and collector electrodes, means to apply said regulated voltage to said emitter electrode, means including a resistive voltage divider having first and second resistors for applying said standard voltage to said base electrode, means for applying a signal from said source of alternating voltage as an additive component to said standard voltage at said base electrode, means for deriving an output signal from said collector electrode, said solid state device being alternately conductive and cutoff, in operative mode and assuming a single state in inoperative mode, a fail-safe capacitor-diode rectifier circuit connected to the output of said solid state device, said output signal causing said fail-safe rectifier circuit to produce a substantial voltage signal level in said operative mode and to produce a negligible voltage signal level in said inoperative mode, and means for applying said voltage signal level to said amplification stage so that said amplification stage is enabled in response to said substantial voltage signal level and disabled in response to said negligible voltage, said monitoring means assuming said inoperative mode upon failure of any non-resistive component thereof.
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2. The system of claim 1 wherein said regulated voltage source includes a zener diode connected in shunt with said photocell, and further including a voltage dropping resistor in series connection with said zener diode.
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3. The system of claim 2 wherein said solid state device has a collector resistor supplying bias current to said collector electrode, said resistive voltage divider being more resistive than said collector resistor, and said collector resistor being more resistive than said voltage dropping resistor.
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4. The system of claim 1 and further including a transistor having its base electrode connected through a resistor to the collector electrode of said solid state device, and arranged to amplify current in said collector electrode to produce said output signal.
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5. The system of claim 1 and further including a transistor having base collector and emitter electrodes and having said regulated voltage connected to its base electrode and the emitter electrode connected to the emitter electrode of said solid state device.
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6. The system of claim 5 wherein said regulated and standard voltages are connected to the base electrodes of said solid state device and said transistor through first and second resistive voltage divider circuits, respectively.
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7. The system of claim 6 and further including a transistor having its base electrode connected through a resistor to the collector electrode of said solid state device and arranged to amplify current in said collector electrode to produce said output signal.
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8. The system of claim 7 wherein said solid state device has a collector resistor supplying bias current to said collector electrode, said resistive voltage divider being more resistive than said collector resistor, and said collector resistor being more resistive than said voltage dropping resistor.
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9. The system of claim 8 and further including a transistor having base collector and emitter electrodes and having said regulated voltage connected to its base electrode and the emitter electrode connected to the emitter electrode of said solid state device.
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10. The system of claim 9 wherein said regulated and standard voltages are connected to the base electrodes of said solid state device and said transistor through first and second resistive voltage divider circuits, respectively.
Specification